Aided by responses, I took a closer look at how the 40mm TWM TB is made:
1) It appears it uses a copy of the Weber butterfly. I measured some distances, got a tangent, and came up with an angle: 77.24 degrees. The butterflies all have "78" stamped on them. Since Paull Abbott let us know that Webers are 78 degrees, and my measurements weren't meticulous, this has to be the deal.
2) The TWM 40s are roughly 50mm OD for the tubular part. That gives a 5mm wall thickness (a bit more, as I measured their bore at only 39.41 with a digital caliper). A 46mm bore would leave the wall at 2mm. Would this be enough? I'd be willing to take that chance, all other things being equal.
3) But then there is injector placement. The injector entrance into the bore is an ellipse. I measured the angle as about 30 degrees.
The tip of the injector is roughly right at the edge of the TB bore.
Doing nothing but boring the TB would leave the injector tip protruding 3mm into the bore.
But just drawing it back would mess up the fuel rail attachment - the rail would have to come up.
On top of this, as the TWM rep warned, it looks like that 3mm would impinge on the lowest O ring seal on the injector.
Mephisto's plan might be an option. The stock fuel rail could attach to a fabricated holder and squirt down through the top of the air filter. TWM thought this risky. Faust did get his soul back in the end, though: Verweile doch, du bist so schoen.
All I want is 50 more HP.